The membrane softening (MS) process and nanofiltration (NF) membranes are revolutionizing the soft water industry and moving water softening from a chemical-based to a largely membrane-based process. This change is evident at the municipal and industrial plant scales of application. The need for improvements in the water softening process at the residential level of application is no less, and no different, than that at the industrial level. The water utility industry is concerned about the cycle of water use (water conservation) and the chemicals added to hard water to convert it into soft water prior to use and disposal (effluent quality degradation). The predominant residential water treatment process used to soften municipal water is ion exchange.
Membrane softening (MS) has been examined for residential use but not significantly adopted because of high water wastage, brine disposal and noncompetitive economics. The MS process produces a de-mineralized permeate stream and a concentrate stream (also called a brine, reject or waste stream). Concentrate disposal may involve injection of the concentrate into a saline aquifer, evaporation, transport by pipeline to a suitable disposal point, or dilution. The disposal of the concentrate in an environmentally appropriate manner usually represents a significant issue and an important cost factor, and it can seriously threaten the feasibility of using MS as a water treatment process.
Difficulty in achieving compliance with wastewater discharge limits is especially pronounced in areas of California with aggressive regional water quality control board programs and in communities dependent upon Colorado River water. Public agencies have not found practical ways to enforce residential water softener regulations, and litigation has arisen over attempts by communities to regulate self-regenerative water softeners. California courts have overturned three local ordinances during the 1990s. State of California Senate Bill 1006, which regulates the installation and use of residential water softening and conditioning appliances, is scheduled to become operative in California on Jan. 1, 2003.
The salt loading impact of residential water softeners on municipal water reclaiming plants has long been a controversial issue. This is because higher salinity increases the treatment costs and reduces the potential for reuse of wastewater for non-potable irrigation purposes. Reclaimed water salinity is typically 250–400 mg/L higher than the originating potable water supply.
The basis for the controversy over the use of inefficient residential water softeners is as follows: The predominant residential water treatment process used to soften municipal water is ion exchange. Part of this treatment process requires the use of a large excess of common salt, which is regularly discharged to the sewer system. Ion exchange processes produce “soft” water by replacing “temporary hardness” ions (calcium, magnesium) with “permanent hardness” ions (usually sodium salts) which are more difficult to subsequently remove from water. In hard water regions facing a future with growing water shortages, the ion exchange approach to water “softening” is becoming regarded as counterproductive and, with respect to its inefficient use in residential water softeners, not in the public interest.
For example, data from a North San Diego Calif. water recycling plant show that de-mineralization is needed to meet the <1000 mg/L TDS requirement for recycled water quality. The total dissolved solids (TDS) of the plant discharge has increased steadily with the use of residential water softening, and future TDS levels are expected to be 1200 mg/l. In average runoff years, the anticipated salinity contributions to the plant discharge are as follows: 600 mg/L TDS from the source water (Colorado River Aqueduct); 350 mg/L TDS from consumptive use; and 80 mg/L TDS attributable to groundwater infiltration. To this 1030 mg/L TDS total must be added a further 50 mg/L TDS from industrial-commercial brines, and 120 mg/L TDS from (discretionary) residential water softener use. Ion exchange use accounts for 170 mg/L TDS of the 200-mg/L TDS excess (violation).
This high salinity contribution from residential water softening must be further examined from the perspective of residential soft water use and the growth of the residential water market. Indoor water use for the San Diego Calif. area accounts for about ⅓rd of the combined indoor and outdoor residential water use. Surveys in parts of the San Diego water supply area also indicate that between 25 and 40 percent of households use an ion exchange water softening system. In volume terms, therefore, it would appear that no more than 8–13 percent of the actual residential water supply is softened by ion exchange. The 120 mg/L TDS (salinity) from residential water softening is an extremely large salinity contribution attributed to a relatively small volume of home water treatment.
Water salinity poses a problem that may in the future be resolved through regulation and public oversight. The problem created by the conflicting use of ion exchange for the treatment of hard water supplies can also be addressed through technological innovation. There is a well-defined opportunity for a new, non-conflicting and regenerative technology to meet residential soft water demand. Parallel desalting is proposed as a means to service residential soft water demand in a competitive, environmentally sensitive and socially responsible manner.
The existing residential use of water softeners by water utility customers in Southern California and other water-limited, hard-water regions varies by locality, with the home use of ion exchange (salt) softeners estimated to be between 10 and 40 percent of the residential customers. Obviously householders purchase water-softening equipment because they want the benefits that derive from access to “soft” water. These benefits include the greater cleansing power of soft water, energy savings from reduced hot water heating costs, reduced scaling and spotting, and less use of soap and detergent. Existing alternatives to the residential use of inefficient self-regenerative water softeners include the physical transportation of ion-exchange brine waste to authorized disposal sites (using portable exchange tanks), and the use of more efficient water softening devices.
It could be argued that individual customer demand for soft water should be met with a residential technology that matches industrial-scale efficiencies and capabilities, while also taking into account the larger treatment and water supply problems faced by water supply utilities as a result of increasing salinity. Industrial-sized membrane desalters are currently used in Southern California to convert brackish groundwater sources to potable standards. However, these plants produce an unusable brine reject stream equal to about 20–25 percent of the volume of water processed. This waste stream, containing a high concentration of salts and other chemicals added before membrane filtration, must be discharged to the ocean or otherwise treated at a disposal facility.
In residential (household) environments, the possibility for developing a high-recovery membrane performance is subject to a number of other important constraints. Living spaces basically preclude the use and/or storage of reagent chemicals for the treatment process, and residential plumbing codes also restrict the use of high operating pressures. High operating pressures are commonly used for ultra pure water production in commercial and industrial membrane applications. A 70–75 percent level of water recovery is, nevertheless, considered by water utility companies to be unacceptable for residential applications.